In recent years, increase of communication data amounts in wireless communications have resulted in, for example, demands for the enhancement of the transmitter power output of base stations. In addition, interest has focused on Doherty power amplification apparatuses including a main amplifier (carrier amplifier) which amplifies an input signal and an auxiliary amplifier (peak amplifier) which amplifies the input signal when the input signal level is higher than a predetermined level, as microwave power amplifiers capable of achieving wide bands and high efficiency.
For example, a gallium nitride (GaN) device may be used as, for example, a power amplification apparatus in a base station. Such GaN devices have wider band gaps and higher mobility than other semiconductor devices (for example, a silicon laterally diffused metal oxide semiconductor (Si-LDMOS), a gallium arsenide field effect transistor (GaAs-FET), and the like), and therefore have excellent high-frequency and high-output characteristics.
In a GaN device, an increase in input power is known to cause a phenomenon, known as an Idq drift, in which an idling current (i.e., drain current on standby during which no signal is input into an amplifier) fluctuates (see, for example, Patent Documents 1 and 2). One cause of the occurrence of the Idq drift is considered to be a threshold value shift.
In other words, an occurrence of a threshold value shift causes an idling current to vary, and furthermore, a fluctuation in idling current causes a gain to fluctuate. Therefore, for example, an amplifier to which a GaN device is applied (power amplification apparatus) has a problem in that gain compensation by a distortion compensation method such as digital pre-distortion (DPD) is precluded, thereby resulting in deterioration of the performance of the distortion compensation.
In order to solve such a problem, for example, there has been known a technique in which an idling current is monitored, and a bias voltage (gate voltage) is applied so that the idling current is within a defined range. In addition, for example, there has been proposed a technique in which an idling current value is determined from the minimum value of a drain current value at the time of RF operation (operation of amplification of high frequency), and a bias voltage (gate bias) is applied to perform distortion compensation based on the idling current value (see, for example, Patent Document 3).
As described above, for example, in a Doherty power amplification apparatus to which a GaN device is applied, there has been proposed a technique, for example, in which an idling current value is determined from the minimum value of a drain current value at the time of RF operation, and a bias voltage is applied to perform distortion compensation based on the idling current value.
However, a main amplifier ordinarily operates in, for example, Class A or Class AB. Therefore, even if an idling current value can be determined from the minimum value of a drain current value, it is difficult to determine the idling current value by a similar technique because an auxiliary amplifier operates in, for example, Class C.
In other words, it is difficult to perform appropriate distortion compensation for a threshold value shift occurring in the auxiliary amplifier of the Doherty power amplification apparatus.
Incidentally, in the past, there have been proposed various Doherty power amplification apparatuses and various power amplification apparatuses to which GaN devices are applied.
Patent Document 1: Japanese Laid-open Patent Publication No. 2010-268393
Patent Document 2: Japanese Laid-open Patent Publication No. 2013-077980
Patent Document 3: Japanese Laid-open Patent Publication No. 2013-247501
Patent Document 4: Japanese Laid-open Patent Publication No. 2012-199746
Patent Document 5: Japanese Laid-open Patent Publication No. 2010-273018